A CRITICAL STUDY OF THE p-DIMETHYLAMINOBENZALDEHYDE METHOD FOR DETERMINING TRYPTOPHANE AS CONTRASTED WITH THE GLYOXYLIC ACID METHOD

Abstract

The p-dimethylaminobenzaldehyde method of estimating tryptophane as modified by May and Rose (1) consisted in incubating a suspension of the protein in a hydrochloric acid solution of the aldehyde reagent at 37" for 6 to 7 days.Casein similarly treated was employed as the standard for color comparison and was assumed to contain 1.5 per cent tryptophane.Holm and Greenbank (2), using pure tryptophane as the standard, found that the bluish violet color reached maximum intensity only after 7 to 9 days incubation and that the relative intensities of the color given by tryptophane and casein indicated that casein contained 2.24 per cent tryptophane.Sullivan, Milone, and Everitt (3) have recently shortened the procedure by heating the mixture of casein and reagent at 85" for 15 minutes and adding dilute HzOz.The color developed by a given amount of casein in this short procedure was found to be identical, in shade and intensity, with that given by the same amount of casein in the long procedure.However, tryptophane could not be employed as the standard in the short procedure because it is unstable in hot acid solution.Casein was recommended therefore as the standard and was said to contain 2.4 per cent tryptophane;i. e., the value obtained by them in the long procedure.Recently we described a modification of the glyoxylic acid method (4) by which a freshly prepared, air-dried sample of casein was found to contain 1.19 per cent tryptophane.The trypto-